Stress distribution around Fe5Si3 and its effect on interface status and mechanical properties of Si3N4 ceramics

Si₃N₄ ceramics with different amount of Fe₅Si₃ were prepared by adding FeSi₂. Residual thermal stress distribution and elastic energy around Fe₅Si₃ particles in various depths were calculated. The interface status between second phase particles and matrix was analyzed in terms of stress and energy. High tangential compressive stresses and low radial tensile stresses were generated along the surface of the ceramics. Elastic strain energy caused by unit interface was high around big particles in deep area of the ceramics. Microcracks are observed around the big Fe₅Si₃ particles. Furthermore, accord to our calculation, microcracks are easily generated around particles in superficial layer of matrix when second phase particles have lower thermal expansion coefficient than the matrix, while microcracks tend to be generated in deep layer of matrix preferentially when the thermal expansion coefficient of second phase particles is higher than matrix. Residual stresses and microcracks around Fe₅Si₃ particles greatly influenced mechanical properties. Fracture toughness of Si₃N₄ ceramics with similar Si₃N₄ particle size distribution increased with the amount of Fe₅Si₃, and fine Fe₅Si₃ particles could enhance the strength of Si₃N₄ ceramics. Si₃N₄ ceramics exceeding 1.2 GPa strength were prepared.